Power converters may generally include switches and one or more capacitors. Such converters are used, for example, to power portable electronic devices and consumer electronics.
A switch-mode power converter is a specific type of power converter that regulates an output voltage or current by switching energy storage elements (i.e. inductors and capacitors) into different electrical configurations using a switch network.
A switched capacitor converter is a type of switch-mode power converter that primarily utilizes capacitors to transfer energy. In such converters, the number of capacitors and switches increases as the transformation ratio increases.
Switches in the switch network are usually active devices that are implemented with transistors. The switch network may be integrated on a single semiconductor substrate or on multiple monolithic semiconductor substrates. Alternatively, the switch network may be formed using discrete devices. Furthermore, because a switch normally carries a large current, it may be composed of numerous smaller switches in parallel.
A common type of switched capacitor converter is a cascade multiplier switched capacitor converter. Two examples of cascade multipliers 16A are shown in FIGS. 1A-1B. The cascade multiplier 16A illustrated in FIG. 1A is often referred to as a Crockcoft-Walton voltage multiplier while the cascade multiplier 16A illustrated in FIG. 1B is often referred to as a Dickson charge pump.
The cascade multiplier 16A illustrated in FIG. 1A receives an input voltage VIN from a voltage source 14 and produces an output voltage VO that is four times the input voltage VIN. Assuming the input voltage VIN is equal to one volt, the operation of the cascade multiplier 16A illustrated in FIG. 1A is described as follows.
A clock with first and second clock intervals generates a voltage at a pump node P1 repeated at a pre-defined frequency. This clock is responsible for controlling the transfer of charge from the voltage source 14 to a load 18. In this example, it takes three iterations of the first and second clock intervals for the initial charge from the voltage source 14 to reach the load 18.
During the first clock interval, the voltage at the pump node P1 is zero volts, and odd labeled diodes D1, D3, D5 conduct current. As a result, charge is transferred from the voltage source 14 to a first pump capacitor CA11, from a first DC capacitor CAZ1 to a second pump capacitor CA12, from a second DC capacitor CAZ2 to a third pump capacitor CA13, and from a third DC capacitor CAZ3 to the load 18.
During the second clock interval following the first clock interval, the voltage at the pump node P1 is one volt, and even labeled diodes D2, D4, D6 conduct current. Consequently, charge is transferred from the first pump capacitor CA11 to the first DC capacitor CAZ1, from the second pump capacitor CA12 to the second DC capacitor CAZ2, and from the third pump capacitor CA13 to the third DC capacitor CAZ3 and the load 18.
The voltage at a first switch node NA11 alternates between one volt and two volts, the voltage at a second switch node NA12 alternates between two volts and three volts, and the voltage at a third switch node NA13 alternates between three volts and four volts. As a result, there is a difference of one volt across each of the pump capacitors CA11-CA13. Meanwhile, the voltage at a first DC node NAZ1 is two volts and the voltage at a second DC node NAZ2 is three volts, resulting in a difference of one volt across each of the DC capacitors CAZ1-CAZ3.
In general, the maximum voltage across each of the pump capacitors CA11-CA13 and DC capacitors CAZ1-CAZ3 in the cascade multiplier 16A is equal to the input voltage VIN, assuming that the peak voltage at the pump node P1 is the input voltage VIN.
Similarly, the cascade multiplier 16A illustrated in FIG. 1B receives an input voltage VIN from a voltage source 14 and produces an output voltage VO that is seven times the input voltage VIN. Assuming the input voltage VIN is equal to one volt, the operation of the cascade multiplier 16A illustrated in FIG. 1B is described as follows.
A first clock generates a voltage at a first pump node P1 and a second clock generates a voltage at a second pump node P2. The first clock and the second clock are one hundred and eighty degrees out of phase with each other, such that they have first and second clock intervals repeated at a pre-defined frequency. These clocks are responsible for controlling the transfer of charge from the voltage source 14 to a load 18. In this example, it takes four iterations of the first and second clock intervals for the initial charge from the voltage source 14 to reach the load 18.
During the first clock interval, the voltage at the first pump node P1 is zero volts, the voltage at the second pump node P2 is one volt, and odd labeled diodes D1, D3, D5, D7 conduct current. As a result, charge is transferred from the voltage source 14 to a first pump capacitor CA11, from a second pump capacitor CA21 to a third pump capacitor CA12, from a fourth pump capacitor CA22 to a fifth pump capacitor CA13, and from a sixth pump capacitor CA23 to the load 18.
During the second clock interval following the first clock interval, the voltage at the first pump node P1 is one volt, the voltage at the second pump node P2 is zero volts, and even labeled diodes D2, D4, D6 conduct current. Consequently, charge is transferred from the first pump capacitor CA11 to the second pump capacitor CA21, from the third pump capacitor CA12 to the fourth pump capacitor CA22, and from the fifth pump capacitor CA13 to the sixth pump capacitor CA23.
The voltage at a first switch node NA11 alternates between one volt and two volts, the voltage at a second switch node NA12 alternates between three volts and four volts, and the voltage at a third switch node NA13 alternates between five volts and six volts. This results in a difference of one volt, two volts, three volts, four volts, five volts, and six volts across pump capacitors CA11, CA21, CA12, CA22, CA13, CA23, respectively. Consequently, there is a different voltage across each pump capacitor.
Assuming, the peak voltage at the first and second pump nodes P1, P2 is the input voltage VIN. The minimum voltage stress is across the first pump capacitor CA11 and equal to the input voltage VIN. While the maximum voltage stress is across the sixth pump capacitor CA23 and equal to six times the input voltage VIN.
It is often desirable for all the capacitors in a cascade multiplier 16A to have the same voltage stress because the same type of capacitor can be used for each capacitor. It is typically more costly to select a separate type of capacitor for each capacitor due to increased supply chain complexity. Furthermore, if the cascade multiplier 16A is monolithically integrated, then it is often more cost effective to include only one type of capacitor with a given voltage rating.
On the other hand, a low voltage capacitor stores less energy than a high voltage capacitor. For example, a cascade multiplier 16A with a series stacked pump capacitor configuration as in FIG. 1A would require a larger amount of total capacitance to achieve the same efficiency as a cascade multiplier 16A with a parallel stacked pump capacitor configuration as in FIG. 1B.
Therefore, it is desirable to have the flexibility to control the distribution of voltage stress among the capacitors along with the maximum and minimum voltage stress across the capacitors.